2000 C-1500 Silverado

Hi, I am new member here and wanted to ask a question about my 2k C-1500, The blower on the A/C and heater, or the selector will not change from the dash a/c vents to the defroster or floor or other selections, what do I need to do to fix this? Thank you for any help.
Kundwg

I had the same issue on my truck before i purchased it from the dealer, They found the problem to be just a plugged filter under the dash. take it out and clean it. hope yours is as simple.